/*
 * Copyright (c) 2013, 2022, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package jdk.graal.compiler.truffle;

import java.util.Collections;
import java.util.LinkedHashMap;
import java.util.Map;

import jdk.graal.compiler.core.common.util.MethodKey;
import jdk.vm.ci.meta.ResolvedJavaMethod;

/**
 * Cache for method or field related data if it cannot be stored directly in the method or field.
 */
public abstract class TruffleElementCache<K, V> {

    private final Map<Object, Object> elementCache;

    /**
     * Value representing a null value in the cache.
     */
    private static final Object NULL_VALUE = new Object();

    @SuppressWarnings("serial")
    protected TruffleElementCache(int maxSize) {
        this.elementCache = Collections.synchronizedMap(new LinkedHashMap<>() {
            @Override
            protected boolean removeEldestEntry(Map.Entry<Object, Object> eldest) {
                return size() > maxSize;
            }
        });
    }

    /**
     * Either use {@link MethodKey} or {@link ResolvedJavaMethod} directly.
     */
    protected abstract Object createKey(K element);

    Object maskNull(Object element) {
        return element == null ? NULL_VALUE : element;
    }

    /**
     * Gets internal representation of a null value as {@code null}.
     */
    Object unmaskNull(Object element) {
        return element == NULL_VALUE ? null : element;
    }

    @SuppressWarnings("unchecked")
    public final V get(K element) {
        Object key = createKey(element);

        /*
         * Do not use Map#computeIfAbsent as Collections.SynchronizedMap#computeIfAbsent blocks
         * readers during the creation of the cached value.
         */
        Object value = elementCache.get(key);
        if (value != null) {
            return (V) unmaskNull(value);
        }
        value = computeValue(element);
        Object existing = elementCache.putIfAbsent(key, maskNull(value));
        if (existing != null) {
            return (V) unmaskNull(existing);
        }
        return (V) value;
    }

    protected abstract V computeValue(K element);

    @Override
    public final String toString() {
        return elementCache.toString();
    }

}
